Hello Matt,
Please try this development build, I have added special tweaks when Sun_SSH is detected and this should improve the compatibility and hopefully resolve the problem you're experiencing
https://oss.azurewebsites.net/testr/dev-builds/ffxp4.2.5.1806.zip
Unzip the flashfxp.exe into your FlashFXP program folder overwriting the existing FlashFXP.exe
I do not have a Sun_SSH SFTP server for testing so my tweaks may or may not be optimal or completely resolve the problems.